The present invention is directed to an improved temperature-resistant, paper base friction material, which when adhesively bonded to a supporting member and used in clutches, bands, brakes, and the like operating in oil, provides a friction element or device having a greater useful life, especially under conditions of high total energy absorption and/or low cooling oil circulation.
Asbestos fibers, by reason of their heat resistance and strength properties, are conventionally used in paper base friction materials for automatic transmission clutch plates and bands, oil brakes and steering clutches operating in a fluid environment. U.S. Pat. No. 3,270,846 discloses such a friction material containing on the order of 5 to 35 percent asbestos fibers. Other similar type paper base friction materials may contain as much as 40-60 percent asbestos fibers.
More recently a paper base friction material which is free of asbestos fibers has been proposed. Thus, according to U.S. Pat. No. 4,045,608, such a paper base friction material comprises a mixture of cellulose fibers, an inorganic absorbent filler and a so-called "retention agent", the sheet being first impregnated with a resin binder followed by an outer coating of an ethyl silicate binder impregnant.
Friction materials of the above-described type, to a large degree, rely upon a porous structure and resiliency to obtain fairly high coefficients of friction in wet friction applications. In those mechanical arrangements providing adequate circulation of cooling oil, the porosity contributes to removal of heat of engagement by allowing penetration of the cooler, external oil into the inner voids of the friction material where it displaces the hotter oil. By so reducing heat build-up at the friction interface, the durability and resistance to fade of the friction material is improved.
Unfortunately, these paper base friction materials, which typically have a porosity of about 60 percent and relatively low heat capacity and thermal conductivity, fail prematurely in conditions of use involving relatively high total energy absorption and/or relatively low cooling oil circulation conditions frequently found, for example, in some multiple disc wet wheel brake and clutch designs.
Friction materials of the foregoing type also are limited as to the degree and type of mating member surface finish against which they are capable of running without substantial loss of durability. Tumbled or ground mating member surface finishes generally no greater than 15 microinch are considered acceptable for low break-in wear and operating wear rates. High density modifications of existing paper base friction materials are capable of running against rougher finishes, but generally have lower dynamic coefficients of friction due to reduced porosity and resiliency.